The postglacial uplift pattern indicated by elevations of ice-marginal glaciomarine deltas in coastal New England, deposited between approximately 15,000 and 14,000 yr B.P. during ice retreat from northeastern Massachusetts into southwestern Maine, is very similar to that previously recorded for glaciolacustrine deltas of similar age from inland areas of New England. Multiple regression analyses of elevations from both sets of deltas show an extremely close fit to tilted flat surfaces that rise 0.852 m/km to the N 28.5°W along the coast and 0.889 m/km to the N 20.5°W in western New England. The close similarity of uplift pattern in areas where elevation data are from different base-level media, along with additional shore-line evidence, indicates (1) that both areas are part of the same crustal postglacial uplift block, (2) that postglacial uplift was delayed until after 14,000 yr B.P., and (3) that little or no eustatic sea-level change occurred between 15,000 and 14,000 yr B.P., during which time the margin of the late Wisconsinan Laurentide ice sheet retreated about 100 km from Boston, Massachusetts, into southwestern Maine. Elevation data from even younger glaciomarine deltas in the coastal area indicate that soon after the ice margin reached southwestern Maine and adjacent New Hampshire (ca, 14,000 yr B.P.), eustatic sea level rose rapidly 7-10 m during the time that the ice margin retreated 5-10 km, which may have occurred during an interval of only 50-100 yr, Our new data not only confirm the delayed postglacial uplift model previously described for western New England, but also indicate that little or no eustatic sea-level change occurred during a substantial period of early deglaciation. However, at about 14,000 yr B.P., sea level rose rapidly. Postglacial uplift in the region apparently began between 14,000 and 13,300 yr B.P., before the retreating ice margin reached eastern Maine.
Although two physically distinct tills of different ages have long been recognized in southern New England, only in the past decade or so has the existence of a similar two-till stratigraphy been recognized in New Hampshire. In southern New England, distinction between the two tills, a lower one and an upper one, initially was based on differences in texture and weathering: the lower compact till has a siltier matrix and an oxidation zone of 10 or more meters; the upper till has a much sandier matrix and an oxidation zone only in the uppermost 1 m. Later, identification of structural features at the contact between the two tills helped to distinguish them. The same stratigraphic relationships are now recognized in all parts of New Hampshire.Excellent exposures at Nash Stream in northern New Hampshire have provided the most complete inland till stratigraphy to date in New England. The tills exposed here are called the Nash Stream (lower) Till and Stratford Mountain (upper) Till; associated deglacial outwash has also been recognized for each of them. The Nash Stream Till is nonoxidized where it is covered by its associated outwash and is oxidized to a depth of 6 to 7 m where it is exposed at the surface. This suggests that a significantly long weathering interval took place before the last ice sheet deposited the Stratford Mountain Till and its associated outwash.The Nash Stream Till, here correlated with the lower till of southern and central New England, may be an early Wisconsinan correlative of the New Sharon Till in Maine and perhaps of the Johnville Till in southeastern Quebec and the Becancour Till in the St. Lawrence Lowland, or it may be even older. The Stratford Mountain Till, of late Wisconsinan Age, is correlated with the surface till throughout New England, and with the Lennoxville Till in Quebec and with at least the upper part of the Gentilly Till in the St. Lawrence Lowland. No middle Wisconsinan units have so far been recognized in New Hampshire and southern New England.
The mode of ice retreat after the maximum advance of the Wisconsinan glacier that last covered New England has been a subject of controversy for more than 100 years. Two major opposing views dur ing most of this period focused on whether recession was characterized by systematic retreat of active glacier ice or by regional stagnation. Difficulty in correlating with the well-established ice-recessional history in the Middle West hampered the discussion in New England. In the last few decades, detailed mapping on large-scale topographic maps has formed the basis for a third model of deglaciation, the morphosequence concept, which contains parts of both previous views. Careful outlining of the distribution and age relationship of melt-water deposits shows that the ice sheet receded by a process of stagnationzone retreat and that the region was deglaciated systematically. End moraines and readvance localities that demonstrate the presence of live ice during retreat in New England are relatively scarce; however, the distribution of such localities indicates that live ice was only a few kilometers from the margin throughout recession. The position, volumes, and especially the altitudes of melt-water deposits suggest that their source material was debris at or near the ice surface. The debris was carried upward from englacial positions to the ice surface along shear planes that resulted from live ice moving over the obstructing stagnant ice at the glacier margin. Analogous shear planes carrying debris have been found in modern valley glaciers.
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